An optical transmission system has to contend with a number of different optical fiber nonlinearities, one of which is commonly referred to as Raman gain. The Raman gain is defined (in dB) as the difference between the power (in dB) of the longest wavelength channel and the power (in dB) of the shortest wavelength channel. The Raman gain, or effect, becomes particularly troublesome when an appreciable level of optical power distributed over a certain range of wavelengths is pumped into an optical fiber. In that instance, the Raman gain is tilted in favor of the channels having the longer wavelengths. As shown graphically in FIG. 1, the Raman effect attenuates the power levels of the optical channels of the lower wavelengths, but increases the power levels of the optical channels of the higher wavelengths by shifting the power from the former channels to the latter channels, as is illustrated in FIG. 1. The Raman gain thus degrades the Signal-to-Noise Ratio (SNR) of the signals in the lower wavelength channels and thus seriously degrades their performance. Experiments indicate that the Raman gain over a wavelength range of 32 nm could be &gt;2.0 dB at 22 dBm total power launched into the transmission fiber for standard single mode fiber over an optical fiber span of 80 kilometers.
The Raman gain/tilt may produce some optical impairments in low power optical systems that have a narrow occupied signal bandwidth, since the total power launched into the fiber would also be low. However, that is not the case for a high power optical system having a relatively wide occupied signal bandwidth. The reason for this is that an optical signal/channel is typically transmitted over an optical fiber at a power level of, for example, &gt;0 dBm to ensure that the optical signal will have a favorable SNR when it arrives at a downstream receiver. Since the power level of each of the optical channels is cumulative, then, in an 80 channel system, the level of the total power launched into the optical fiber at the output of an optical amplifier could be more than 20 dBm.